Process for foam production using dissolved under pressure carbon dioxide

ABSTRACT

A process is described for the continuous production of polyurethane block foam using carbon dioxide which is physically dissolved under pressure as a foaming agent, wherein before the polyol and isocyanate components are mixed the carbon dioxide is dissolved in the polyol component and air or nitrogen is dissolved in the isocyanate component, both components are fed to a mixing chamber in which a pressure prevails which is 70 to 150% of the solution pressure of the CO 2  in the polyol component, the isocyanate component is fed to the mixing chamber at a pressure of at least 30 bar and is injected therein with depressurization down to the mixing chamber pressure, wherein air or nitrogen in an amount of at least 1 g per kg CO 2  is dissolved in the isocyanate component, and after emerging from the mixing chamber the mixture is depressurization to atmospheric pressure.

FIELD OF THE INVENTION

The present invention relates to a process and an apparatus for theproduction of foams by means of carbon dioxide dissolved under pressureas a foaming agent, wherein the composition to be foamed is mixed underpressure with what is preferably liquid carbon dioxide and issubsequently depressurised with the formation of foam. Liquid startingmaterials for plastics, which harden to form the foamed plastic due toan addition polymerisation or condensation polymerisation reaction whichsets in after foaming, are used in particular as the foamablecompositions. This invention relates in particular to polyurethanefoamed materials.

BACKGROUND OF THE INVENTION

The use of physically dissolved carbon dioxide as a foaming agent in theproduction of polyurethane foams is known from GB-A 803 771, U.S. Pat.No. 3,181,199 and U.S. Pat. No. 3,184,419. However, these known, priorproposals have not resulted in an industrial use, since the foamstructure produced was very non-uniform; in particular, the foamcomprised large voids. The reason for this is considered to be that thedissolved carbon dioxide has a pronounced tendency to remain insolution, even when the mixture which reacts to form polyurethane issupersaturated with carbon dioxide. In order to release the carbondioxide during or after depressurisation it is necessary to provide seedbubbles which promote the controlled release of the carbon dioxideduring depressurisation.

This problem is also known to occur when other foaming agents are used,such as low molecular weight hydrocarbons, chlorofluorocarbons,methylene chloride or water (chemical evolution of carbon dioxide by thereaction of the isocyanate with water). In these situations, bubbleseeds have been provided by dispersing finely distributed air and/ornitrogen in at least one of the components of the mixture which reactsto form polyurethane.

When carbon dioxide which is physically dissolved under pressure is usedas the foaming agent, it has similarly been proposed according to EP-A645 226 that a nitrogen be introduced as a nucleating gas into themixing chamber for the polyol and isocyanate components. In the courseof this procedure, the amount of nitrogen has to be increased comparedwith conventional mixing processes which operate substantially at normalpressure, so that it corresponds to the prevailing pressure. However,polyurethane foams produced in this manner still have an unsatisfactoryfoam structure, apparently because the seed bubble structure is notsufficiently fine and uniform.

It has also been proposed according to DE-A 44 22 568.7 that highshearing forces be produced at the depressurisation element fornucleating the release of CO₂. Even though very good grades of foam areobtained according to this proposal using high carbon dioxide contentsbetween 4 and 6% by weight and with depressurisation from acorrespondingly high pressure, at lower carbon dioxide contents in thereactive mixture the supersaturation of the reactive mixture which isproduced on depressurisation is not sufficient to provide uniformly goodgrades of foam by means of high shearing forces. Another disadvantage isthat the mixing chamber pressure has to be maintained very closely abovethe solution pressure of the carbon dioxide, which constitutes anobstacle to a volume flow control procedure.

SUMMARY OF THE INVENTION

The object of the present invention is to improve the foam qualities ofmulti-component foamed materials using carbon dioxide dissolved underpressure, by producing a very uniform bubble seed structure.

It has been found that the mixing chamber pressure can be varied withinwide limits, in the interest of effecting mass flow control of thecomponents, if air or nitrogen is firstly dissolved as a nucleatingagent in one of the (main) components and carbon dioxide is secondlydissolved in the other (main) component, and if the component containingthe nucleating agent is injected under high pressure into the componentcontaining carbon dioxide.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 shows a diagram of the process of the present invention.

FIG. 2 shows a preferred embodiment of the depressurizing element of thepresent invention.

FIG. 3 shows a diagram of the process of the present invention for thecontinuous production of block foam.

DETAILED DESCRIPTION OF THE INVENTION

The present invention accordingly relates to a process for thecontinuous production of polyurethane block foam using carbon dioxidewhich is physically dissolved under pressure as a foaming agent, whereinbefore the polyol and isocyanate components are mixed, the carbondioxide is dissolved in the polyol component and air or nitrogen isdissolved in the isocyanate component, both components are fed to amixing chamber in which a pressure prevails which is 70 to 150%,preferably 80 to 130%, of the solution pressure of the CO, in the polyolcomponent, the isocyanate component is fed to the mixing chamber at apressure of at least 10 bars above the pressure prevailing in the mixingchamber, and is injected therein with depressurisation down to themixing chamber pressure, wherein air or nitrogen in an amount of atleast 1 g, preferably 3 to 6 g per kg CO₂, is dissolved in theisocyanate component, and after emerging from the mixing chamber themixture is depressurised to atmospheric pressure.

Preferably the pressure of the isocyanate component prior tointroduction into the mixing chamber is above 30 bars, particularlypreferred between 40 and 100 bars.

The amount of air dissolved in the isocyanate is preferably 4 to 6 g perkg CO,; in the case of nitrogen it is 3 to 5 g per kg CO₂. The additionof air or nitrogen can be effected on the intake side of the meteringpump. With large amounts of air or nitrogen, however, addition on theintake side results in metering problems at the isocyanate pump. In thissituation, addition on the delivery side, i.e. downstream of themetering pump, is suitable, In this connection it is important that thedistance between the static mixer and the mixing head iso-nozzle is longenough so that a dwell time results during which the air or nitrogengoes into solution completely.

Carbon dioxide is preferably used as the foaming agent in an amount of1.5 to 6 parts by weight per 100 parts by weight polyol, correspondingto a mixing chamber pressure of about 4 to 30 bar. The solution partialpressure of CO₂ in the mixture is about 3 to 14 bar, and depending onthe formulation, on the raw materials used, and on the proportions ofair dissolved therein due to transport and storages variations of around0.3 bar (at low CO₂ contents) to 2 bar (at high CO₂ contents) arepossible. Since all of the carbon dioxide is introduced into the mixingchamber dissolved in the polyol component, the polyol pressure beforeits introduction into the mixing chamber is at least 5 bar (1.5 parts byweight CO₂) to at least 20 bar (6 parts by weight CO₂).

Customary formulations, in which 50 to 65 parts by weight isocyanate areused for 100 parts by weight polyol, are assumed in this respect. In theevent that the formulation is synthesised from prepolymer components,this results in different pressure ratios for the components and themixture, corresponding to the mixture ratio of the components which isthen necessary.

After mixing of the components is complete (assuming that theproportions of gas released in the bubble seeds are also dissolved), thesum of the solution partial pressures of carbon dioxide and air ornitrogen is less than the mixing chamber pressure. The bubble seedsproduced on injecting the isocyanate into the mixing chamber areapparently not redissolved during the short dwell time of 1 to 2 secondsuntil the depressurisation of the reactive mixture to atmosphericpressure.

According to the invention, very uniform bubble seeds are produced in anamount of 250,000 to 500,000 per g of reactive mixture, with a seedbubble diameter between 10 and 30 g in the reactive mixture.

Outlet orifices which are of small cross-section in at least onedimension, such as apertures or perforated plates, and which impose aresistance to flow on the reactive mixture which is sufficient tomaintain the mixing chamber pressure, are suitable as a depressurisationelement for the depressurisation of the reactive mixture. A plurality ofperforated plates disposed in series is preferred, wherein theperforated plates adjacent to the mixing chamber have a sufficientlysmall free cross-section for maintaining the mixing chamber pressure andat least one perforated plate is provided remote from the mixing chamberwhich has a considerably larger free cross-section for example, so thatthe outlet velocity of the reactive mixture is reduced on its passagethrough the last perforated plate. The invention is explained in moredetail below with reference to the accompanying FIGS. 1 to 3.

FIG. 1 shows an agitating mixer head 3 which is customarily used inpolyurethane foam technology, to which isocyanate from isocyanate supplyvessel 35 and polyol from polyol supply vessel 36 are fed via lines 31and 32, respectively. The isocyanate emerging from isocyanate supplyvessel 35 is brought to a pressure of 95 bar, for example, via pressurepump 33. Before it enters the static mixer 37, the isocyanate is chargedwith air, as indicated by arrow 39, which is intimately mixed with theisocyanate in the static mixer. The inlet of isocyanate line 31 into themixing chamber 3 is equipped with a nozzle by means of which theisocyanate is injected into the mixing chamber at a pressure P_(i) of 90bar, for example, and is depressurised to the mixing chamber pressureP_(m) of 14 bar for example. The dwell time between the static mixer 37and the mixing chamber injection is about 10 seconds. The polyolemerging from polyol supply vessel 36 is brought to a pressure of 30bar, for example, by means of pressure pump 34. Liquid carbon dioxidewhich is cooled to -20° C. is injected into the polyol line, asindicated by arrow 40, before the latter enters the static mixer 38, andis mixed in the static mixer 38. For example, the amount of carbondioxide which is dissolved in the polyol upstream of the static mixermay be 4 parts by weight per 100 parts by weight polyol, correspondingto a solution partial pressure of 13 bar. The pressure P_(p) in polyolfeed line 32 is then about 20 to 23 bar.

When polyol line 32 enters the mixing chamber 3, the polyol isdepressurised by means of a pressure relief valve to the mixing chamberpressure P_(m) of 14 bar. Apart from carbon dioxide, the polyol may alsocontain about 4.5 parts by weight of water per 100 parts of polyol as anadditional chemical foaming agent. According to one customaryformulation, 60 parts of isocyanate are fed to the mixing chamber 3 per100 parts of polyol, wherein the isocyanate may contain 0.33 g air/kgisocyanate.

The depressurisation element for depressurising the reactive mixture toambient pressure is disposed directly at the outlet of the mixingchamber 3. FIG. 2 shows a particularly preferred embodiment of thedepressurisation element 4. This consists of a housing 41 which isclosed by one or more perforated pressure plates 42, which comprise amultiplicity of holes with a diameter 0.1 to 0.2 mm, for example,wherein the free cross-sectional area of all the holes amounts to 1 to5% of the plate area for example. A further perforated plate 43 isdisposed upstream of the perforated pressure plates 42, and serves toreduce the velocity of the reactive mixture passing through theperforated pressure plates. The velocityreducing plate 43 also has holeswith diameters of 0.1 to 0.2 mm, but the free cross-sectional area ofall the through-holes amounts to 10 to 30%. The perforated plates areheld by a retaining flange 44 and have spacer rings 45, so that they areat a spacing of 0.5 to 2 mm. The perforated plates 42 and 43 arepreferably convex in the direction of flow as indicated by the dashedlines 42a and 43a. Directly it has passed through the perforated plates42 and 43, the dissolved carbon dioxide is released in the bubble seedsproduced in the mixing chamber. The reactive mixture thereby foams verystrongly, as indicated by the foam contour 5.

FIG. 3 illustrates the process for the continuous production of blockfoam. The foam 5 emerging from the depressurisation device 4 isdeposited on a lower laminating film 2 which travels in conjunction withthe conveyor belt 1. A barrier 6 which is disposed transversely on theconveyor belt prevents the foam from flowing off in the oppositedirection to that of the conveyor belt. The upper conveyor belt 8 is fedvia a roller 7. Lateral rollers 9 serve to feed the lateral laminatingfilm 10. It is also indicated that other auxiliary materials andmodifying substances can be fed to the mixing head via an additionalfeed line 30.

With the formulation given in the description of FIG. 1, a very uniformfoam is produced, which has a bulk density of 15 kg/m³ and very uniformfoam bubbles. The foam contains 18 foam bubbles/cm.

Although the invention has been described in detail in the foregoing forthe purpose of illustration, it is to be understood that such detail issolely for that purpose and that variations can be made therein by thoseskilled in the art without departing from the spirit and scope of theinvention except as it may be limited by the claims.

What is claimed is:
 1. A process for the continuous production ofpolyurethane block using carbon dioxide which is physically dissolvedunder pressure as a foaming agent, comprising the steps of dissolvingthe carbon dioxide in a polyol component and dissolving air or nitrogenin an isocyanate component before mixing said polyol and isocyanatecomponents; feeding both said components into a mixing chamber in whicha pressure prevails which is 70 to 150% of the solution pressure of thecarbon dioxide in said polyol component; said isocyanate component isfed into said mixing chamber at a pressure of at least 10 bars above thepressure prevailing in said mixing chamber and is injected therein withdepressurization down to the pressure of said mixing chamber; air ornitrogen in an amount of at least 1 g per kg of carbon dioxide isdissolved in said isocyanate component, and after emerging from saidmixing chamber, said mixture is depressurized to atmospheric pressure.2. A process according to claim 1, whereby 0.5 to 6 parts by weight ofcarbon dioxide, with respect to 100 parts by weight of polyol, aredissolved in the polyol component, and the mixing chamber pressure is 2to 30 bar.
 3. A process according to claim 1, whereby air in an amountof 4 to 6 g per kg CO₂ is dissolved in the isocyanate component.
 4. Aprocess according to claim 1, whereby a pressure prevails in the mixingchamber which corresponds to 80 to 130% of the solution pressure of theCO₂ in the polyol component.
 5. A process according to of claim 1,whereby 2 to 6 parts by weight of carbon dioxide, with respect to 100parts by weight of polyol, are dissolved in the polyol component.